Recently, a nitride semiconductor serving as a group III-V compound semiconductor represented by GaN is expected to be applied to a switching element. Especially, as for the nitride semiconductor, compared with silicon, its band gap is as large as 3.4 eV, its breakdown electric field is 10 times higher, and its electron saturation speed is 2.5 times faster, so that it has characteristics suitable for a power device.
More specifically, a switching element having a heterostructure of GaN/AlGaN on a sapphire substrate or the like has been proposed, for example (refer to a Patent Document 1, for example). As for this switching element, two-dimensional electron gas (2DEG) of no fewer than 1×1013 cm−2 can be generated due to spontaneous polarization caused by asymmetry of a GaN crystal structure (wurtzite type) in a C axis direction, and polarization provided by a piezo effect resulting from lattice mismatch of AlGaN and GaN. This switching element switches conducting state/non-conducting state between predetermined electrodes by controlling this two-dimensional electron gas.
The switching element having the above structure will be described specifically with reference to FIGS. 6 to 8. FIG. 6 is a cross-sectional view showing a structure of a conventional switching element. FIG. 7 is a cross-sectional view showing an off state of the conventional switching element shown in FIG. 6. FIG. 8 is a cross-sectional view showing an on state of the conventional switching element shown in FIG. 6.
As shown in FIG. 6, a switching element 100 includes a substrate 101, a buffer layer 102 formed on an upper surface of the substrate 101, an electron running layer 103 formed on an upper surface of the buffer layer 102 and composed of undoped GaN, an electron supplying layer 104 formed on an upper surface of the electron running layer 103 and composed of AlGaN, a source electrode 105 formed on an upper surface of the electron supplying layer 104, a drain electrode 106 formed on the upper surface of the electron supplying layer 104, and a gate electrode 107 formed on the upper surface of the electron supplying layer 104 and arranged between the source electrode 105 and the drain electrode 106. In addition, the switching element 100 is a normally-on type.
As for the switching element 100, when a potential of the gate electrode 107 is equal to a potential (set to 0 V) of the source electrode 105, or the gate electrode 107 is open, it is switched to a state (on state) in which two-dimensional electron gas 108 is generated in an interface of the electron running layer 103 with the electron supplying layer 104. At this time, when a potential of the drain electrode 106 is higher than the potential of the source electrode 105 (when it is a positive potential), a current flows between the drain electrode 106 and the source electrode 105.
Meanwhile, as for the switching element 100, when the potential of the gate electrode 107 is lower than the potential (set to 0 V) of the source electrode 105 by a predetermined value or more (when it is a negative potential), it is switched to a state (off state) in which the two-dimensional electron gas 108 is not generated in the interface of the electron supplying layer 104 with the electron running layer 103, just below the gate electrode 107. In this state, a current does not flow between the drain electrode 106 and the source electrode 105.
As shown in FIG. 7, when the switching element 100 is switched to the off state, a depletion region 109 is formed just below the gate electrode 107. At this time, as for the switching element 100 for the power device, a high potential difference (about several hundred V corresponding to a power supply voltage, for example) is generated between the drain electrode 106 and the source electrode 105. As a result, a high electric field is generated in the vicinity of the gate electrode 107 on the drain electrode 106 side, and electrons and holes are generated due to impact ionization. Thus, the generated electrons 110 are trapped in a level such as a level caused by a nitrogen defect in the surface (upper surface) of the electron supplying layer 104.
When the switching element 100 is switched from the off state shown in FIG. 7 to the on state, as shown in FIG. 8, the electrons 110 trapped in the surface of the electron supplying layer 104 are held for a predetermined time (as long as several seconds to several minutes, for example). The electron 110 exerts a repulsive force (Coulomb force) on an electron in the two-dimensional electron gas 108, and prevents the current from flowing between the drain electrode 106 and the source electrode 105. This is a phenomenon called a “collapse phenomenon”, and on-resistance of the switching element 100 is increased due to this phenomenon, so that high-speed switching is difficult to perform, which is a problem.
A structure to prevent this collapse phenomenon is disclosed in a Patent Document 2. This structure will be described with reference to FIG. 9. FIG. 9 is a cross-sectional view showing a structure of a conventional switching element.
As shown in FIG. 9, a switching element 200 includes a substrate 201, a buffer layer 202 formed on an upper surface of the substrate 201, an electron running layer 203 formed on an upper surface of the buffer layer 202 and composed of undoped GaN, an electron supplying layer 204 formed on an upper surface of the electron running layer 203 and composed of AlGaN, a source electrode 205 partially formed on the upper surface of the electron running layer 203, a drain electrode 206 partially formed on the upper surface of the electron running layer 203, a gate electrode 207 formed on the upper surface of the electron supplying layer 204 and arranged between the source electrode 205 and the drain electrode 206, and a passivation layer 211 formed on the upper surface of the electron supplying layer 204, and arranged between the gate electrode 207 and the source electrode 205 and between the gate electrode 207 and the drain electrode 206.
As for this switching element 200, since the passivation layer 211 composed of nitride is provided on the upper surface of the electron supplying layer 204, a nitrogen defect is prevented from being generated in the surface (upper surface) of the electron supplying layer 204. In addition, since the switching element 200 has a structure (field plate structure) in which the gate electrode 207 extends at least toward the drain electrode 206, an electric field is reduced from being generated in the vicinity of the gate electrode 207 on the drain electrode 206 side, so that the above-described impact ionization is prevented from being generated.